A while ago, I saw the message "You feel vibrations from deep below...". The Destroyer spawned and promptly destroyed me (since I had just entered Hardmode and only had Molten Armor).
Why did the Destroyer choose to spawn right then? What are the conditions for it, the Twins, and Skeletron Prime to spawn? No bosses have naturally spawned since then, and I am wondering what makes them spawn.
I have gotten a mechanical worm, a mechanical skull, and a mechanical eye from enemies, too. When do these drop? Are they supposed to be the natural spawns, instead of the boss just coming?